1. Field of the Invention
The present invention relates to a spot joining method and a spot joining device for spot joining of works made of aluminum alloy or the like.
2. Description of the Related Art
Conventionally, as a method for spot joining of plates made of aluminum alloy or the like, there have been a resistance spot welding method, a mechanical fastening method using rivets, and the like.
In the resistance spot welding method, there exist problems in running cost and quality control, including a need for a power supply of a large capacity in spot joining, short life duration of welding electrodes due to contamination or wear-out, and a need for utilities such as water and air in addition to a welding power.
In the resistance spot welding method, since indentations remain on joined portions, this method is undesirable when applied to outer plates for automobile because its external appearance is lessened.
In particular, in the resistance spot joining of works made of the aluminum alloy, since the electrodes significantly wear out, the ability to perform continuous spotting is low. When current is conducted, it flows through spots close to a spot to be conductive, that is, current is shunted. Hence, it is impossible to place the spots close to one another. For this reason, it is impossible to obtain required strength.
Meanwhile, riveting requires a number of rivets as wear-out members.
The present invention has been developed for obviating the above-described problem and an object of the present invention is to provide a spot joining method and a spot joining device in which quality control is easily performed, running cost is improved, and no indentations remain on joined portions.
According to the present invention, there is provided a spot joining method comprising the steps of: rotating a joining tool having a pin at a tip end portion thereof around an axis of the joining tool with the pin pressed against a predetermined joint spot of lapped works to be joined, the pin being protruded along the axis; inserting the pin into the predetermined joint spot of the lapped works heated and softened due to friction heat; stirring portions of the lapped works that are in the vicinity of the predetermined joint spot by using the rotating pin and fusing the lapped works at the predetermined joint spot; and pulling out the joining tool along the axis, thereby performing spot joining of the lapped works at the predetermined joint spot.
In this method, two works to be joined are lapped and placed. The joining tool is rotated at a high speed and the pin at the tip end portion thereof is pressed against the joint spot of the lapped works to be joined. Thereby, the joint spot is heated and softened due to friction heat and the pin is inserted thereinto. By further rotating the tool, portions of the works around the pin plastically flow and are stirred. The two works are fused at the joint spot. After pulling out the joining tool, the softened works are solidified and the two works are spot-joined at the joint spot.
Thus, in the spot joining method of the present invention, the joining tool is rotated to generate friction heat for joining the works. Therefore, compared with the resistance spot welding, spot joining can be carried out with a power of a significantly smaller capacity. In addition, utilities other than the power are unnecessary, and the joining tool can be used over a long period of time without the need for maintenance because the joining tool is neither contaminated nor worn out. Therefore, running cost is reduced and management is easily made. Unlike the mechanical fastening method using rivets, since fastening members are unnecessary in the spot joining method of the present invention, cost and weight are not increased.
In this method, a receiving member having a flat receiving face on which the lapped works are placed is provided opposite to the tip end portion of the joining tool to receive a pressing force from the joining tool pushing against the lapped works.
In spot joining, for example, two works to be joined are lapped and placed on the receiving member. From above of the works, the joining tool is rotated and pressed against the works to be joined. The works are supported on the receiving member such that a face of one of the works is pressed against the flat face of the receiving member, and spot joining is performed from the opposite side of the receiving member. A hole is formed on the works as a result of pin insertion, but the face pressed against the flat face of the receiving member is kept flat because the pin is inserted from the opposite side of the receiving member and does not reach the face. After joining, this work can be used as an outer plate.
According to the present invention, there is also provided a spot joining device comprising: a joining tool having a pin at a tip end portion thereof, the pin being protruded along an axis of the joining tool; a first motor for rotating the joining tool around the axis thereof; and a second motor for moving the joining tool along the axis thereof, wherein the joining tool is moved along the axis by the second motor while the joining tool is rotated by the first motor, the pin is pressed against a predetermined joint spot of lapped works to be joined and is inserted into the predetermined joint spot heated and softened due to friction heat, portions of the lapped works that are in the vicinity of the predetermined joint spot are stirred by using the rotating pin, the lapped works are fused at the predetermined joint spot, and the joining tool is pulled out along the axis by the second motor, whereby the lapped works are spot-joined at the predetermined joint spot.
According to this device, the joining tool is rotated by the first motor and is moved along the axis by the second motor. Thereby, plastic flow occurs due to the generated friction heat as described above. Thus, a plurality of works can be spot-joined at joint spots.
In this device, the first motor is one of an induction motor and a servo motor and the second motor is the servo motor.
According to this device, by using the induction motor or the servo motor as the motor for rotating the joining tool around the axis, the joining tool can be rotated at a high speed. Also, by using the servo motor as the motor for moving the joining tool along the axis, a joining depth can be controlled with high precision.
These objects, as well as other objects, features and advantages of the invention will become more apparent to those skilled in the art from the following description with reference to the accompanying drawings.